old 911's and 914's use the dash light wiring for the 'exciter' current for the alt. check the fuse for the alt and clean the fuse contacts. you are not charging for some reason.

key switch going bad?

what does a volt meter across the battery show running and not running.

50 amp fuse blown on the firewall?

do not get a 'marine battery' it does not have the amperage that you need for a car.

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g-wagen